Deformation characterization of additively manufactured Inconel 718

How do heat treatments affect the mechanical properties of additively manufactured alloys?

Supriya Koul, Akihiro Kushima, and their colleagues at University of Central Florida published work using their Hummingbird Scientific in-situ biasing nano-manipulator TEM sample holder to perform in-situ tensile deformation on additively manufactured Inconel 718 exposed to different heat treatments. The samples were printed using selective laser melting and push-to-pull devices were created using focused-ion beam.

Deformation TEM videos of a) As-printed Inconel 718, b) As-printed Inconcel 718 after heating to 800°C in the TEM, and c) Solution heat treated Inconel 718. d) Relationship between crack length and notch opening in deformed specimens with inset showing push-to-pull device. Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of the Microscopy Society of America

Nanoscale lamellae were extracted from as-printed and solution heat-treated (1065°C for 1 hr, 720°C for 8 hr, and 650°C for 7 hr) samples. In-situ tensile deformation was performed on an as-printed lamella, a lamella heat-treated in situ at 800°C in the TEM, as well as the solution heat-treated lamella, and the fracture process was captured. A crack developed slowly in the as-printed sample, with a linear relationship between notch opening and crack length indicative of ductile fracture, while the sample heated in situ showed a rapid increase in crack length indicative of brittle fracture. The solution heat-treated sample showed slower crack growth and retained its strength and hardness, indicating slower ductile fracture impeded by the needle-like δ-phase precipitates that formed during heat treatment. The in-situ exploration of heat treatment and effect on alloy strength and fracture mechanisms can also be applied to other material systems and manufacturing processes.

Reference: Supriya Koul, Le Zhou, Omar Ahmed, Yongho Sohn, Tengfei Jiang and Akihiro Kushima, Microscopy and Microanalysis 27 (2) 250-256 (2021) DOI: 10.1017/S1431927621000052

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